Flotation is a three phase system of solids, water and air. The valuable minerals are usually made hydrophobic by addition of suitable reagents. Frother reagents are added to the slurry to stabilize bubbles and reduce surface tension at slurry surface so that bubble swarms do not coalesce and burst. Reagents referred to as collectors are added to the slurry to make the valuable particles hydrophobic so that they will attach to the dispersed air bubbles and slowly rise to form a stable froth zone at the cell surface.
Air is fed into the slurry in a flotation vessel to infuse and disperse bubbles into the slurry. When the particles of the valuable substance come into contact with the bubbles they are attached to the bubbles and rise upwards to the surface of the slurry to form a foam bed (herein called as a froth phase) above the free surface of the slurry (slurry phase). The froth can then be removed from the vessel by overflow over an overflow lip into the froth launder for further processing. A froth washing device has been arranged to disperse wash water into the froth phase in the flotation vessel before the overflow to wash out undesirable hydrophilic fine particles entrained from the slurry phase to the froth phase.
Flotation methods are described e.g. in prior art documents U.S. Pat. Nos. 7,328,806, 7,163,105, 6,793,069, 5,601,703, 5,814,210, 4,804,460, 2,756,877, 2,182,442, 2,369,401 and 1,952,727.
The prior art document, U.S. Pat. No. 1,952,727 discloses a method and apparatus in which water is applied to the froth by spray pipes which extend transversely of the flotation cell above the froth. The spray pipes are perforated in such a manner that fine streams or small droplets of water may be directed against the surface of the froth at points removed from the overflow lip. When a gentle spray of pure water is applied by the spray pipes, the water tends to pass downwardly through the layer of froth and in so doing it becomes substituted in the films of the bubbles with which it comes into contact for the liquid of which the films were originally formed. The displaced liquid together with the undesirable entrained gangue particles and other materials contained therein passes downwardly into the slurry body thereby increasing the quality of the concentrate flowing over the launder lip.
Such technology has been used for many years in both base metals and the coal industry to improve product quality. The majority of wash water devices transfer clean water into the froth zone of a flotation cell to remove entrained hydrophilic gangue particles (size <10 μm) that have no commercial value. The presence of these fine gangue particles can be a major factor in reducing the grade of the concentrate from the flotation circuit i.e. MgO minerals in Nickel concentrate, as or silica (ash content) in coal fines.
There are many wash water devices and these range from fine sprays located within the froth zone (e.g. U.S. Pat. Nos. 5,814,210, 5,167,798, 4,981,582) to a series of static perforated stainless steel water trays that sit above the froth and transfer water via a series of 3 mm diameter holes into the froth surface. This wash water penetrates the froth zone and slowly passes downwards through the froth layer by gravity and replaces the existing entrained water and fine hydrophilic gangue thus improving the overall quality of the froth concentrate.
The most common problems with the known technology has been the constant need for cleaning the blocked sprays and the conduits. The quality of wash water has always been a major issue in the industry and it is almost impossible to guarantee that a process disturbance will not occur that will result in the process water becoming dirty and contaminated with the fine solids. Also a lack of maintenance access to the flotation cells has long been an ongoing problem with static wash water trays. Hence there is a need for a device that can substantially ameliorate or overcome some or all of these problems with the prior art.